1. Field of the Invention
This invention relates to a device and a method for compensating light beam fluctuations in various light beam application devices for recording, display, working, measurement, calculation, etc., and further an optical device equipped with said device, particularly to a light beam fluctuation compensating device and method for effecting compensation so that the intensity center of the irradiating beam will invariably be directed to a predetermined spot and further it relates to an optical device equipped with said device.
2. Related Background Art
In the prior art, as a method for measuring light absorption characteristics of a sample, there is a method of determining light absorption characteristics from transmittance or reflectance. However, when a sample is irradiated with light, scattering of light occurs in addition to transmission and reflection of light, and it becomes important for evaluation of light absorption characteristics to measure directly the absorbed component of light in order to effect further high precision measurement.
As the method for direct measurement of the absorbed component of light, there may be included Photoacoustic Spectroscopy (PAS) which is a measuring method utilizing the phenomenon that the light energy absorbed by a sample is converted intermittently to heat through a radiationless relaxation process when irradiated intermittently with light and the Photothermal Radiometry (PTR).
The PAS method may be classified into the microphone method and the piezoelectric device method according to the detector. In the microphone method, the sample is required to be placed in a sample chamber which is hermetically sealed, while arrangement of the detector and the sample becomes the problem in the piezoelectric device method. Thus, neither method is suitable for measurement of a sample under special environment such as measurement of a thin film spread on a liquid surface. On the other hand, the PTR method employs an IR-ray detector, and therefore has the drawback that it is susceptible to fluctuation in the atmosphere such as water vapor, etc.
On the other hand, also as the method for measuring directly the absorbed component of light, there is a method called Photothermal Deflection Spectroscopy (PDS). The PDS method utilizes the phenomenon that, with heat generation by light absorption of the sample, a temperature distribution is caused in and around the sample to change the refractive index, whereby the light incident thereon is deflected. More specifically, by irradiating exciting light which changes the refractive index by causing a temperature distribution by heat generation at the time of light absorption and a probe light for measuring the deflection caused thereby on the measuring site of a sample, the light absorption characteristics of a sample are measured from the wavelength of the exciting light and the deflection of the probe light. This method can afford setting of a sample and a detection system independently of each other and therefore suitable for measurement at the site or for remote measurement. The PDS method includes the two types of the transverse type and the collinear type depending on arrangement of the exciting light and the probe light, and either method measures the deflection of the probe light corresponding to the exciting light absorption of the sample and position sensitive detector (PSD) is frequently used as the detector.
FIG. 20A is an example of the collinear type, in which the exciting light 111 emitted from the exciting light source 110 is made into an intermittent light beam by the chopper 112 and condensed by the lens 134 to be irradiated on the sample 104. On the other hand, the probe light 106 emitted from the probe light source 105 is permitted to pass through the region of the sample 104 irradiated with the excited light 111 by means of the optical path controller 117 comprising the lens 135 and mirror, etc., and reaches the detector 107, and the deflection when deflected as shown by the broken line is measured. FIG. 20B is an example of the transverse type, which is the same as the collinear type except that the probe light 106 is irradiated in parallel to the surface of the sample 104.
The PDS method can be theoretically dealt with by solving the thermal conduction within the sample, and the deflection measured as the deflection angle .phi. is propotional to the excited light intensity, temperature coefficient of refractive index (n/T), the temperature gradient in the region where the probe light passes (T/x), etc. The items proportional to the light absorption coefficient of the sample are included in (T/x). As for (n/T), it can take either positive or negative value depending on the sample, and this means that the angle may be either positive and negative.
FIG. 21 is a longitudinal sectional view showing a structural example of one dimensional PSD. In FIG. 21, the one dimensional PSD constitutes a uniform resistance layer 34 of P layer on the surface of a flat plate silicon 33, provided at both sides with electrodes X.sub.1 and X.sub.2, having the common electrode 36 provided on the N layer 35 on the back side.
FIG. 22 shows schematically its actuation principle. The charges formed by light corresponding to the incident position of the light Q reach the above resistance layer 34 as photocurrent corresponding to its energy and split in inverse proportion to the distance from the position Q to the take-out electrodes X.sub.1, X.sub.2 and outputted from the respective electrodes. If the photocurrent by the incident light is defined as I.sub.L, the currents I.sub.x1, I.sub.x2 become as follows: EQU I.sub.x1 =I.sub.L.R.sub.x2 /(R.sub.x1 +R.sub.x2) EQU I.sub.x2 =I.sub.L.R.sub.x2 /(R.sub.x1 +R.sub.x2)
Further, since the resistance between X.sub.1 and X.sub.2 maintains uniform distribution, the following respective formulae are valid between the resistance between X.sub.1 and X.sub.2 and the length L: EQU R.sub.x1 +R.sub.x2 =L EQU R.sub.x1 =X EQU R.sub.x2 =L-X
For this reason, the signal taken out from the respective electrodes can be represented by L and X as follows: EQU I.sub.x1 =I.sub.L.(L-X)/L EQU I.sub.x2 I.sub.L.X/L
Thus, informations of incident position of light and light intensity can be obtained at the electrodes of X.sub.1 and X.sub.2.
Further, by calculating a ratio of the difference between I.sub.x1 and I.sub.x2 relative to the sum thereof and defining it as the position signal P, the following formula can be obtained: ##EQU1## Corresponding to x=0 to L, the position signals irrelevant to the light intensity change can be continuously obtained as follows: ##EQU2##
The above description concerns the one-dimensional case, and the two-dimensional position detector may be also considered similarly and the position signals can be determined from the block diagram of the actuation circuit shown in FIG. 23.
Here, from the actuation principle of PSD, when there are light incidence of two points or more the position signals weighted in proportion to the respective light intensities can be obtained. Also, in the case when the light flux is expanded, the position signal as the center of gravity of light intensity can be obtained. The center of gravity of light intensity, or center of intensity distribution, of the light beams, is defined as the point where the integral values of intensity of the distributed light become equal in all directions from this point.
However, when the PDS method as described above is applied as such for measurement of a thin film spread on the liquid surface, there is the inconvenience caused by the fact that the thin film of the sample is extremely thin. That is, in measurement using such PSD, the fluctuation in emission angle of the light source itself has a great effect on the measurement precision. Particularly, in measurement by use of gas laser, it has been impossible to perform positional detection at high precision, and it has been difficult to obtain desired light absorption characteristics.
FIG. 24 is a schematic illustration showing one example of the light irradiation device of the prior art. In FIG. 24, the light beam which is emitted from the light source 101 and reaches the target 103 through the lens 102 takes the pathway along the course 104 shown by the solid line or the course 105 depending on the fluctuation in the emitted direction of the light source 101, whereby the positional deviation occurs at the target irradiated position.
FIG. 25A shows fluctuation with time of the positional deviation of light beam, in which the axis of ordinate indicates the positional deviation W and the axis of abscissa the time t. In the Figure, the solid line indicates the position where the intensity of the light beam is at the peak, the dotted line the light beam width., and the positional deviation W is fluctuated randomly with time so that the average value over a long time becomes 0.
FIG. 25B is the results of optical information recording carried out in spots by condensing the above light beam showing the recorded pattern when recording was effected while deviating linearly the position X. As is apparent in this Figure, this shows the problem that the positional precision of the record is lowered by recording of the pattern, which is to be recorded linearly on the X axis, at the position corresponding to the positional deviation W.
FIG. 25C is a graph showing a trace line when laser minute working is performed with the same light beam. This shows the problem that the working line which should be drawn as a straight line on the X axis becomes wavy due to the beam fluctuation.
Such problems can be solved by stabilization of the spot center position, if a long irradiation time is scheduled and measurement is awaited before the light beam settles at the average position. However, such a solution will take too long time and therefore not suitable for high speed recording or working.